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(#) Insecure HostnameVerifier

!!! WARNING: Insecure HostnameVerifier
   This is a warning.

Id
:   `BadHostnameVerifier`
Summary
:   Insecure HostnameVerifier
Severity
:   Warning
Category
:   Security
Platform
:   Any
Vendor
:   Android Open Source Project
Feedback
:   https://issuetracker.google.com/issues/new?component=192708
Since
:   2.0.0 (April 2016)
Affects
:   Kotlin and Java files
Editing
:   This check runs on the fly in the IDE editor
See
:   https://goo.gle/BadHostnameVerifier
Implementation
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-checks/src/main/java/com/android/tools/lint/checks/BadHostnameVerifierDetector.java)
Tests
:   [Source Code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/BadHostnameVerifierDetectorTest.java)

This check looks for implementations of `HostnameVerifier` whose
`verify` method always returns true (thus trusting any hostname) which
could result in insecure network traffic caused by trusting arbitrary
hostnames in TLS/SSL certificates presented by peers.

(##) Example

Here is an example of lint warnings produced by this check:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~text
src/test/pkg/InsecureHostnameVerifier.java:9:Warning: verify always
returns true, which could cause insecure network traffic due to trusting
TLS/SSL server certificates for wrong hostnames [BadHostnameVerifier]
    public boolean verify(String hostname, SSLSession session) {
                   ------
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Here is the source file referenced above:

`src/test/pkg/InsecureHostnameVerifier.java`:
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~java linenumbers
package test.pkg;

import javax.net.ssl.HostnameVerifier;
import javax.net.ssl.SSLSession;

public abstract class InsecureHostnameVerifier  {
    HostnameVerifier allowAll = new HostnameVerifier() {
        @Override
        public boolean verify(String hostname, SSLSession session) {
            return true;
        }
    };

    HostnameVerifier allowAll2 = new HostnameVerifier() {
        @Override
        public boolean verify(String hostname, SSLSession session) {
            boolean returnValue = true;
            if (true) {
                int irrelevant = 5;
                if (irrelevant > 6) {
                    return returnValue;
                }
            }
            return returnValue;
        }
    };

    HostnameVerifier unknown = new HostnameVerifier() {
        @Override
        public boolean verify(String hostname, SSLSession session) {
            boolean returnValue = true;
            if (hostname.contains("something")) {
                returnValue = false;
            }
            return returnValue;
        }
    };
}

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can also visit the
[source code](https://cs.android.com/android-studio/platform/tools/base/+/mirror-goog-studio-main:lint/libs/lint-tests/src/test/java/com/android/tools/lint/checks/BadHostnameVerifierDetectorTest.java)
for the unit tests for this check to see additional scenarios.

The above example was automatically extracted from the first unit test
found for this lint check, `BadHostnameVerifierDetector.testBroken`.
To report a problem with this extracted sample, visit
https://issuetracker.google.com/issues/new?component=192708.

(##) Suppressing

You can suppress false positives using one of the following mechanisms:

* Using a suppression annotation like this on the enclosing
  element:

  ```kt
  // Kotlin
  @Suppress("BadHostnameVerifier")
  fun method() {
     problematicStatement()
  }
  ```

  or

  ```java
  // Java
  @SuppressWarnings("BadHostnameVerifier")
  void method() {
     problematicStatement();
  }
  ```

* Using a suppression comment like this on the line above:

  ```kt
  //noinspection BadHostnameVerifier
  problematicStatement()
  ```

* Using a special `lint.xml` file in the source tree which turns off
  the check in that folder and any sub folder. A simple file might look
  like this:
  ```xml
  &lt;?xml version="1.0" encoding="UTF-8"?&gt;
  &lt;lint&gt;
      &lt;issue id="BadHostnameVerifier" severity="ignore" /&gt;
  &lt;/lint&gt;
  ```
  Instead of `ignore` you can also change the severity here, for
  example from `error` to `warning`. You can find additional
  documentation on how to filter issues by path, regular expression and
  so on
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/lintxml.md.html).

* In Gradle projects, using the DSL syntax to configure lint. For
  example, you can use something like
  ```gradle
  lintOptions {
      disable 'BadHostnameVerifier'
  }
  ```
  In Android projects this should be nested inside an `android { }`
  block.

* For manual invocations of `lint`, using the `--ignore` flag:
  ```
  $ lint --ignore BadHostnameVerifier ...`
  ```

* Last, but not least, using baselines, as discussed
  [here](https://googlesamples.github.io/android-custom-lint-rules/usage/baselines.md.html).

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